The present invention relates generally to metal-joining techniques, and more particularly provides a unique method of bonding the aluminum (or other nonferrous metal) laminae of a fluidic device or the like.
Fluidic devices--those devices which utilize a high velocity fluid jet to perform various sensing or control functions--are customarily formed from a multiplicity of stainless steel laminae which are bonded together by a brazing process. The laminae have various openings formed therein which, in the assembled fluidic device body, collectively define rather intricate internal passages through which the operating fluid is flowed. To bond the stainless steel laminae together the facing surfaces thereof are first coated with a layer of copper-tin plating. The plated laminae are then stacked, in a predetermined aligned relationship, with the copper-tin surfaces of adjacent laminae in intimate contact. The aligned stack is then heated to approximately 1825.degree. F. to fuse the adjacent pairs of copper-tin coatings, thereby bonding the plated laminae into the finished fluidic device.
Because of its lighter weight and lower cost, aluminum is potentially a very desirable alternate laminate material for fluidic devices of the type just described. However, for a variety of reasons, previous attempts to bond thin aluminum fluidic laminae have not resulted in an entirely satisfactory end product. The primary difficulty encountered in applying conventional bonding techniques to aluminum fluidic laminae arises from two design criteria which must be adhered to in manufacturing fluidic devices.
First, the internal fluid passages in the assembled device are extremely sensitive to even minute obstructions. Accordingly, the bonding material cannot be permitted to seep into any of the passages during the bonding process. Secondly, even a small degree of laminae warpage during the bonding process can correspondingly distort the precisely configured passages and seriously diminish the device's overall accuracy.
The first of these design criteria effectively rules out the simple expedient of sweat soldering the stacked aluminum laminae together since it would be extremely difficult, if not impossible, to keep the flowing solder out of the internal fluidic passages, while still obtaining a uniform bond between all surfaces of the intricate laminae. Moreover, the difficulties of soldering aluminum components are well known in the metal-joining art. Specifically, conventional solder materials simply do not adhere well to aluminum surfaces.
The copper-tin plating method used to bond stainless steel laminae is equally unsuitable since the plating material, like conventional solder, does not satisfactorily adhere to aluminum. Additionally, even if such copper-tin plating could be suitably applied to aluminum, its melting point is on the order of 1825.degree. F.--well above the approximately 1140.degree. F. melting point of aluminum. Simply stated, the aluminum laminae would melt before the copper-tin bonding plating could be melted and joined.
Conventional aluminum-to-aluminum bonding techniques such as "cladding" and diffusion bonding have also proven undesirable when applied to fluidic laminae. The cladding method consists of making the aluminum laminae from aluminum sheet having cladding material on its side surfaces which has a melting point (approximately 1000.degree. F.) that is only slightly lower than the 1140.degree. F. melting point of the underlying laminae. The cladded laminae are then positioned in an aligned stack, and the stack is heated to the cladding melting point to fuse the facing cladding layers. This particular process, however, is quite expensive because of the relatively high cost of the cladding material and the long heat up and hold cycle required to achieve the actual bonding. Additionally, warpage of the laminae stack is very difficult to avoid since the 1000.degree. F. cladding melting point is considerably above the approximately 600.degree. F. softening temperature of the aluminum laminae.
Diffusion bonding is another conventional approach to intersecuring aluminum laminae. Under this technique, the uncoated laminae are positioned in face-to-face contact and are subjected, for a predetermined time period, to a sufficiently high temperature and pressure to cause direct aluminum-to-aluminum fusion between the facing laminae. In the fluidics area, however, this bonding method has proven, like the others discussed above, to be less than wholly satisfactory.
First, diffusion bonding of aluminum is both a relatively difficult and time-consuming process. The difficulty arises because the facing aluminum surfaces must be kept from oxidizing before or during the actual diffusion bonding--yet aluminum has the well known propensity for extremely rapid oxidation. To counter this tendency, rather intricate preparation techniques must be employed. Moreover, the time-temperature-pressure interrelationship in the diffusion process must be carefully controlled to prevent warpage of the laminae stack.
More specifically, the diffusion pressure and temperature must be kept at sufficiently low levels to avoid distortion of the stack and its internal passages. With these necessary upper limits on temperature and pressure, the time required to achieve adequate diffusion is greatly increased--in the ordinary instance to several hours for a given laminae stack. This combination of long diffusion "holding periods" and surface preparation difficulties greatly increase the overall manufacturing expense of the fluidic device, thereby rendering diffusion bonding economically unfeasible in most fluidics applications.
From the foregoing it can be seen that a need exists for an aluminum laminate bonding technique which can be economically utilized to accurately produce fluidic devices of the type described. It is accordingly an object of the present invention to provide such a technique.